The invention relates to a method for generating entries for a database, which database is destined for supporting a positioning of a mobile terminal, in particular a hybrid positioning of a mobile terminal. The invention relates equally to a unit realizing such a method and to a system realizing such a method.
In a hybrid positioning of a mobile terminal, data from a main positioning system, e.g. a satellite based positioning system, are combined with data from a cellular network in order to determine the position of the mobile terminal.
A well known satellite based positioning system is GPS (Global Positioning System). In GPS, code modulated signals are transmitted by several satellites that orbit the earth and received by GPS receivers of which the current position is to be determined. Each of the satellites transmits two microwave carrier signals. One of these carrier signals L1 is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier signal is modulated by each satellite with a different C/A (Coarse Acquisition) Code known at the receivers. Thus, different channels are obtained for the transmission by the different satellites. The C/A code, which is spreading the spectrum over a 1 MHz bandwidth, is repeated every 1023 chips, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with the navigation information at a bit rate of 50 bit/s. The navigation information comprises in particular ephemeris data. Ephemeris parameters describe short sections of the orbit of the respective satellite. Based on these ephemeris parameters, an algorithm can estimate the position and the velocity of the satellite for any time of about 2-4 hours during which the satellite is in the respective described section. Ephemeris data also comprise clock correction parameters which indicate the current deviation of the satellite clock versus a general GPS time.
Further, a time-of-week TOW count is reported every six seconds as another part of the navigation message.
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and a tracking unit of the receiver detects and tracks the channels used by different satellites based on the different comprised C/A codes. The receiver first determines the time of transmission of the ranging code transmitted by each satellite. Usually, the estimated time of transmission is composed of two components. A first component is the TOW count extracted from the decoded navigation message in the signals from the satellite, which has a precision of six seconds. A second component is based on counting the epochs and chips from the time at which the bits indicating the TOW are received in the tracking unit of the receiver. The epoch and chip count provides the receiver with the milliseconds and sub-milliseconds of the time of transmission of specific received bits. A detected epoch edge also indicates the code phase of a received signal.
Based on the time of transmission and the measured time of arrival TOA of the ranging code at the receiver, the time of flight TOF required by the ranging code to propagate from the satellite to the receiver is determined. By multiplying this TOF with the speed of light, it is converted to the distance between the receiver and the respective satellite. The computed distance between a specific satellite and a receiver is called pseudo-range, because the general GPS time is not accurately known in the receiver. The computed distances and the estimated positions of the satellites then permit a calculation of the current position of the receiver, since the receiver is located at an intersection of the pseudo-ranges from a set of satellites.
In weak signal conditions, however, GPS positioning cannot be carried out in a standalone fashion in the receiver. Assistance of some kind is needed to recover the positioning capability. If the GPS receiver is part of or connected to a mobile terminal operating in a cellular communication network, the simplest form of GPS assistance is to deliver navigation data over the cellular network to the receiver. Usually, missing navigation data is the key element why positioning cannot be maintained or initiated in weak signal condition for a long period.
A more sophisticated form of assisting the receiver is a delivery of the exact GPS time to the receiver. Exact time is needed e.g. to improve the sensitivity of the receiver. However, along the exact time also a reference location of some quality is basically mandatory. The reference location, i.e. a known position near to the expected location of the receiver, is needed for calculating geometrical distances between the satellites and the receiver. The calculated distances are then used for predicting navigation data bit edges and C/A-code phases, in order to improve the sensitivity of the receiver and to speed up the signal acquisition.
The availability of a reference location is thus a key factor for some time recovery and sensitivity improvement methods in assisted GPS. If the GPS receiver is part of or connected to a mobile terminal operating in a cellular communication network, the coordinates of the cell in which the mobile terminal is known to be located could be used as reference location. However, while a cellular communication network usually provides a mobile terminal with an identification of the cell in which it is currently located, this identification does not contain information about the geographical location of the cell. The geographical location information of the cells in a cellular network is usually controlled by the network operators. Thus, in order to obtain a reference location from the network, it is necessary to poll the location from the network, which might be time consuming. Especially in the case of emergency calls a delay in signal acquisition may be critical. Moreover, the network operators offer the position information usually as a chargeable service to their subscribers, unless it is required for an emergency call.
If an available reference location is too far from the receiver, it decreases the possibilities to assist GPS hardware in acquisition and in tracking and prohibits the use of some time recovery methods in the case that the time assistance is not exact. Thus, it is also useful to know the reliability of a provided reference position, i.e. the maximum distance from the current position of the receiver to the provided reference location. If the accuracy of the reference location is known to be good enough for some application, the reference position can even be used as such, and GPS is not required at all.
In conventional positioning systems, the accuracy of the reference location is either provided by a cellular communication network or not available at all.
Similar problems may also arise with other positioning systems than GPS.
In order to avoid the necessity of polling the cellular network each time a reference location is required, European patent application EP 1 237 009 A2 introduced the idea of a cell location database. The proposed database is used to store the geographical position of cells. In the Global System for Mobile Communication (GSM), each cell has a unique Cell Global Identity (CGI) identification. When a CGI has once been associated with a geographical position, this position can be stored and used as reference location also later on, whenever the receiver is in the coverage area of the cell with this particular CGI. The database can be stored in the non-volatile memory of a mobile terminal or be downloaded from a network independently from the operator of the cellular network, e.g. using the Wireless Application Protocol (WAP). The cited patent application, which is incorporated by reference herein, also provides a detailed description on how the geographical information in the database can be used as reference location in the positioning of a mobile terminal. It does not specify, however, how the geographical information can be estimated.
It is an object of the invention to enable the generation of information on cells of a cellular network for a database which is to be used for a positioning of a mobile terminal.
A method is proposed which comprises as a first step calculating at least one position of a mobile terminal in a cell of a cellular network. This at least one position can be calculated for instance based on satellite signals using a satellite positioning system. It can be calculated equally, however, using some other positioning method like a network-based positioning method, e.g. E-OTD (enhanced observed time difference), or such positioning solutions as WLAN (wireless local area network) positioning or Bluetooth(trademark) positioning. In a second step, geographical information on the cell is determined based on the at least one calculated position of the mobile terminal in the cell. The geographical information can comprise for instance coordinates, i.e. latitude and longitude values, which belong to a location within the cell. Finally, the determined geographical information is provided together with an identification of the cell, e.g. a CGI, for storage in the database.
The mobile terminal can be any device which is suited to exchange signals with a cellular network. The proposed method or parts of the proposed method can be implemented in the mobile terminal which comprises processing means to this end, or in another unit external to the mobile terminal which comprises processing means to this end. The method can be implemented for example in a network unit which receives the required information for the processing from the mobile terminal. Further, the mobile terminal may comprise a receiver for receiving satellite signals, which can be used for determining the at least one position of the mobile terminal, e.g. a GPS receiver, or it may be connectable to such a receiver.
Further, a system is proposed which comprises a mobile terminal with communication means for communicating with a cellular network, a database destined for supporting a positioning of said mobile terminal, and processing means for realizing the proposed method. These processing means can be distributed in any suitable way to the receiver, the mobile terminal and, if desired, to an additional network unit.
A cellular network usually provides a mobile terminal attached to the network with an identification of each cell of the network which is entered by the mobile terminal. The invention proceeds from the idea that an available information on the identity of a cell of a cellular network in which a mobile terminal is currently located can be combined with position information for this mobile terminal which is obtained by some positioning method. Since the mobile terminal is known to be located in a specific cell when a positioning is performed, the data of a recorded position can be used for geographical information on the cell for future visits of the cell, when it is stored in a database. In case several positions are available for one cell, a plurality of positions can be evaluated to determine the geographical information, in order to obtain a reference location which is as close as possible to the center of the cell. The location of the center of the cell is the preferred reference location, since the maximum distance of a mobile terminal located in the cell to some reference location in the cell is minimal, when the reference location correspond to the center of the cell.
It is to be noted that a mobile terminal will often be located within several cells at the same time, since it will usually be able to receive signals not only from the serving cell but equally from some of the neighboring cells, which could also function as serving cell. In such a case, the mobile terminal is able to receive an identification of all of these cells, e.g. the CGIs of all of these cells. Therefore, the mobile terminal may create database information for all of these cells according to the invention, not only for the current serving cell.
A mobile terminal which has access to the database for which the geographical information is provided is able to perform an assisted positioning under weak signaling conditions without having to poll the cellular network for reference location. The information stored in the database can also be used by itself for a rough positioning when the resolution of the cell locations is good enough for a desired purpose.
It is an advantage of the invention that it allows to generate the cell position information without requiring any assistance data from the cellular network. As a result, a positioning of a mobile terminal which is based on the available cell identity is not dependent on position information from cellular networks and their operators, and costs related to the delivery of reference locations by the operators can be avoided completely. Further, the time to first fix in a hybrid positioning might be reduced.
The determination of the geographical information can be based on a single calculated position, on two calculated positions or on a plurality of calculated positions. In case a plurality of positions are to be determined at different points of time, these positions can be determined for example in regular intervals.
Additionally to the geographical information, a cell range can be estimated and provided for storage in the database. The cell range can consist in particular in the radius of the cell. Such a cell range can, but does not have to be estimated equally based on calculated positions, e.g. the same positions which are used for determining the geographical information.
In an advantageous embodiment of the invention, the determined geographical information and the estimated cell range can be updated. Usually, the serving cell and those neighboring cells which are xe2x80x9cheardxe2x80x9d by the mobile terminal change quite frequently, especially when there are many cells and there is a lot of communication traffic in the area. In such a situation, the position information for a specific cell is gathered during a very short period of time and in a very limited area, even if in reality the cell is rather large. The mobile terminal may leave the cell and enter it again later on, and each time at least one position is calculated for the cell. If all these pieces of position information are combined to provide the estimate of the geographical information and of the cell range, the result will be much closer to the real geographical information and the real cell range than if the estimates are based only on one gathering handled separately. This aspect of the invention is of particular advantage for large cells, but useful for cells of all sizes.
In one approach, the positions calculated during each serving period of a cell may be stored and be used together with positions calculated during later serving periods of the same cell for determining the geographical information and/or the cell range anew. In another, preferred approach, only the geographical information and the cell range determined for a specific serving period are stored. The positions calculated during later serving periods of the same cell are then combined with the stored geographical information and the stored cell range. The latter option has the advantage that it does not consume any extra memory for storing the calculated positions.
For the storage of the provided information in a database, the database may comprise different kinds of data structures.
Preferably, the data structure is hierarchical or comprises a hash table. Both approaches support a fast search for the information on a particular cell.
The database can be stored for example in a memory of the mobile terminal, which enables a particular fast access to the stored data. Alternatively or additionally, it could also be stored in a unit external to the mobile terminal. This external unit can be for example a network server, like an Internet server, in which the determined geographical location and possibly a cell range is generated, or to which this information is reported. A network server has the advantage that the available memory for storing the information can be larger. A network server could also maintain a global database for collecting geographical information provided by different mobile terminals. From this database, subsets can then be delivered upon request to various mobile terminals.
The method according to the invention can be implemented in particular by software.
Other objects, features and advantages of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not drawn to scale and that they are merely intended to conceptually illustrate the structures and procedures described herein.